1. Technical Field
This invention relates generally to submersible well pumping systems. This invention relates particularly to a positive displacement pumping system enclosed in a housing and comprising a multi-chamber hydraulically driven diaphragm pump, with an improved hydraulically actuated switching mechanism.
2. Description of the Background Art
Hydraulically driven diaphragm pumps are positive displacement pumps which are nearly immune to the effects of sand in the pumped fluid because the pressure generating elements are isolated from the pumped fluid by a flexible diaphragm. In well pump applications, this type of pump is driven by a self contained, closed hydraulic system, activated by an electric or hydraulic motor where the pump, closed hydraulic system, and the motor are enclosed in a common housing and submerged in a well. There are many examples of this type of well pump in the patent literature, but currently none are in use as well pumps because of high cost and/or poor reliability. In well pump applications, the key design feature is the switching mechanism used to redirect or reverse the flow of working fluid from the fluid source, referred to as the auxiliary pump, to the working fluid sub-chambers. The reversal of the flow causes the pumped fluid to move into and out of-pumped fluid sub-chambers through check valves, accomplishing the pumping action.
U.S. Pat. No. 2,435,179 discloses a hydraulically driven diaphragm pump which uses a hydraulically actuated valve to reverse the flow of working fluid. The valve is driven by differential pressure between the fluid inside the working diaphragm (working fluid) and the fluid outside the working diaphragm (pumped fluid). Normally, no differential pressure exists between the two volumes. The pump creates the differential pressure required to reverse the pump by completely filling the diaphragm, causing it to stretch after it is completely full. The amount of pressure generated is limited by the strength of diaphragm material and has the disadvantage of creating diaphragm stress, which can lead to premature diaphragm failure. To maximize diaphragm life, this differential pressure must be limited to the lowest level possible.
The ""179 patent uses two sets of diaphragms, one set to control the valve, and the other set to achieve pumping. The pumping diaphragms are located inside the pumping chambers, and the control diaphragms are located between the working fluid inside the pumping chambers and the pump outlet. The external surfaces of the working and control diaphragms are separated by an outlet check valve, creating the possibility that the external pressure would be higher on the control diaphragm due the presence of the checkvalve. If the inlet pressure is higher than the outlet pressure (a common occurrence in well pumps) the pump will not operate and could be damaged. This situation commonly occurs when the pump is installed in a standing fluid column, before fluid has a chance to equalize by flowing through the pump checkvalves. This arrangement also limits the usefulness of the pump by limiting it to use in conjunction with a large diameter liner rather then a more conventional, smaller diameter drop pipe.
A more significant problem occurs in low volume applications. The nature of the pump requires that the hydraulically actuated valve be driven by the same pressure source controlled by the valve, which causes the valve driving force to be released when the valve transverses an intermediate position between states. In low volume applications, this single valve can stop in an intermediate position before it has completely reversed the pump. This can cause the pump to either dither (rapid but incomplete movement of the working fluid in one direction), or go into a mode where half the flow is directed into each chamber or stops, which causes the pump to stop functioning.
Other problems will occur with the valve setup disclosed in the ""179 patent. For example, the control diaphragm is acting directly on a tappet, leading to fluid accumulation between the diaphragm and the tappet, which in turn leads to diaphragm failure unless measures are taken to relieve the fluid. For these and other reasons, the pump described in the ""179 patent has never been used in a practical application. This patent application addresses those shortcomings and describes a practical well pump with in improved control valve.
U.S. Pat. No. 2,961,966 discloses another method to reverse the flow of working fluid by reversing the direction of rotation of the electric motor driving the auxiliary pump. That patent discloses a method to sense the differential pressure between the working fluid and the pumped fluid to activate the electrical braking and reversal of the electric motor driving the auxiliary pump. That method also leads to diaphragm stress because differential pressure is required across the diaphragm to actuate the sensor. In addition motor reversal requires very complex electronics. Although theoretically possible, in practice the complexity of that method leads to high expense and unreliable operation due to the difficulty of controlling and reversing the electric motor in a downhole environment.
U.S. Pat. No. 6,017,198 discloses another method to reverse the flow of working fluid, namely the use of sensors and electronics to detect the fact that the diaphragm is full, and reverse the direction of flow by using an electrically actuated valve. This method works very well, but requires relatively complex electronics and a connection into the main power cable. Sealing electronics and power cables against high ambient pressure environments found in wells is expensive and can lead to premature failures of the pump due to high ambient pressure related electrical shorts.
Another unexpected problem can occur when pumping in certain environments, namely the accumulation of gas or the corrosion of the internal workings of the pump due to saturation of a corrosive gas through the diaphragm into the pump workings. Loss of working fluid and a related problem of working fluid contamination of the pumped fluid can also be problems, especially in water well applications where oil in the drinking water is not acceptable. This patent application describes two methods to address these problems increasing the applicability of the pump into more restrictive settings.
A pumping system, like the one disclosed herein, which combines the high reliability and ease of installation of a submersible centrifugal pump with the high efficiency in low flow-high pressure applications of a positive displacement pump constitutes a significant advancement in the state of the relevant art.
The primary pumping system of the invention comprises an axially elongated housing having a diameter less than the bore hole of the well, a pump with a plurality of pumping chambers of fixed volume, each pumping chamber is further subdivided by a flexible diaphragm into two sub-chambers, a working fluid sub-chamber and a pumped fluid sub-chamber, typically made of rubber. Each pumped fluid sub-chamber is connected to the bore hole of the well through a check valve which allows well fluid to flow into the pumped fluid sub-chamber but prevents flow in the reverse direction. Likewise, each pumped fluid sub-chamber is connected through a check valve which allows the well fluid to flow out of the pumped fluid sub-chamber to the pump outlet but prevents flow in the reverse direction. Such an arrangement allows well fluid to flow through the pumped fluid subchambers, thereby moving the pumped fluid from the bore hole of the well to the pump outlet and eventually to the surface. The movement of well fluid into and out of the pumped fluid sub-chambers is caused by the insertion or withdrawal of working fluid into and out of the working fluid sub-chambers. The movement of working fluid is caused by a closed hydraulic system which forces working fluid into one or more working fluid sub-chambers while simultaneously withdrawing working fluid from one or more opposite working fluid sub-chambers. The closed hydraulic system comprises an auxiliary pump, a main valve, a plurality of control valves, a plurality of control chambers, the working fluid subchambers, and passageways. The passageways extend from the auxiliary pump to the main valve, from the main valve to the control valves, and from the main valve to the working fluid sub-chambers. The control chambers are connected to the control valves. The auxiliary pump, which can be a piston pump, gear pump, centrifugal pump or any type of pump that produces the required flow rates and pressures, provides inlet and outlet flows of working fluid. The main valve is connected to the inlet and to the outlet of the auxiliary pump and to two sets of working fluid sub-chambers, each set comprising roughly equal displacement.
The main valve has two states. In the first state, the inlet of the auxiliary pump is connected to one set of working fluid sub-chambers, and the outlet of the auxiliary pump is connected to the other set of working fluid sub-chambers. In the second state, the main valve connects the set of working fluid sub-chambers previously connected to the input of the auxiliary pump, to the outlet of the auxiliary pump, and connects the input of the auxiliary pump to the set of working fluid sub-chambers previously connected to the output of the auxiliary pump.
The main valve is driven between states by pilot pressure applied to two control ports. The valve is bi-directional, that is it will move between two states under the influence of pilot pressure in either direction, the direction of change determined by which port is under the higher pressure. Both control ports are normally connected to the low pressure (input) of the auxiliary pump through the control valves. One control valve is connected to each of the two control ports. Each control valve is also connected to the appropriate working fluid sub-chamber and control chamber. The control chambers are volumes of working fluid, located in the vicinity of the matching pumped fluid and working fluid sub-chambers, having rigid boundaries except where it is separated from the pumped fluid sub-chamber by a flexible diaphragm. When the pressure in the working fluid sub-chamber exceeds the pressure in a matching control chamber by a predetermined amount (due the filling of the working fluid sub-chamber), the control valve opens and allows flow from the working fluid sub-chamber to the control port on the main valve. This creates differential pressure between the control ports and drives the main valve to the opposite state.
The main valve must be able to complete the movement between the two states while the switching of the main valve is eliminating the differential pressure activating the control valve. If the main valve stops before the center position is passed, the valve will return to the original state and create a dithering, or rapid cycling condition, eventually leading to pump failure. Another failure mode occurs when the main valve stops short of full switching.
To prevent this, an energy storage element combined with hysteresis and/or a latch is added to the system to create a bistable main valve. The energy storage element stores energy in a spring, compressed gas, kinetic energy of a moving mass, or by lifting a mass. In a pump, the most convenient method to store energy is in a spring. In this system the pumping diaphragm provides a convenient spring to store the energy needed to shift the main valve, when differential pressure expands the pumping diaphragm at the point at which the main valve is ready to shift. The pumping diaphragm acts like an accumulator, prolonging the pressure in the system even after the main valve has cut off and reversed the flow of fluid into the working fluid sub-chamber. This stored energy maintains the differential pressure across the pumping diaphragm and maintains the control valve in the activated condition while the main valve completes the transition between the two states.
This effect can be enhanced by providing a detent latch on the main valve that will prevent the transition of the main valve until sufficient pressure and flow are present at the control ports. This latch provides two beneficial effects. First, it eliminates the tendency of the main valve to move in response to transient signals such as water hammer, common to this type of pump. This prevents the valve from getting hung up under certain conditions of operation. Second, it increases the speed and force of the transition of the main valve by allowing the control valve to fully open before any movement of the main valve. This sharpens the transition increasing the possibility that the valve will fully shift under all conditions.
Hysteresis in the two control valves also helps to assure the main valve completes the transition between states. Hysteresis in this context is the tendency of the control valve to actuate at one pressure, and unactuate at a lower pressure. Hysteresis is a normally undesirable characteristic found in most valves, and is caused by fluid damping or internal friction in the valve. The amount of Hysteresis can be controlled and increased by adding more damping or friction. Hysteresis acts similarly to the energy storage effect, increasing the amount of time the control valve is open after the main valve starts transitioning, allowing the main valve to complete the transition before the control valve closes.
Other design features are important to assure proper operation. More reliable operation is achieved if the volume of the main valve control ports is maintained as small as possible. To achieve this the stroke of the main valve should also be maintained as small as possible. Attention should also be paid to passageway lengths and diameters to minimize pressure drops in the system.
A small amount of fluid is moved from the working fluid sub-chamber to the control chamber when the valve is switched. To cycle the fluid from the control chamber to the working fluid sub-chamber, a check valve allowing flow from the control chamber to the working fluid sub-chamber or a small orifice between the chambers can be used.
The auxiliary pump is driven by a prime mover that can be an AC or DC rotary electric motor, a AC or DC linear motor, a hydraulic motor or mechanical actuation from the surface. In the preferred embodiment of the invention, the prime mover is contained in the same housing as the pump, and is powered electrically. The pump may be connected to the motor in such a way that they share a common fluid supply, that is the same fluid is used in the electric motor as is used as the working fluid in the pump. In this arrangement, the fluid input of the auxiliary pump is connected to the electric motor fluid volume. This arrangement has the advantage of reducing the possibility of failure due to working fluid leakage around shaft seals, because the shaft seal between the pump and the motor is eliminated, which results in no moving seals between the working fluid and the well fluid. The fluid in the electric motor volume and working fluid in the closed hydraulic system in the pump expand and contract with temperature and pressure and must be equalized with the pump inlet to prevent pump and/or electric motor failure. Because the electric motor volume and the closed hydraulic system in the pump constitute one fluid volume, the working fluid sub-chambers compensate for this expansion and contraction for both the electric motor volume and the closed hydraulic system in the pump, eliminating the need for a separate expansion compensation for each volume.
Another favorable arrangement is achieved by separating the electric motor fluid and the pump working fluid volumes through a shaft seal between the auxiliary pump and the electric motor. In this arrangement, different fluids with different properties can be used in each volume. To reduce the likelihood of failures, the shaft seal is situated between the motor fluid and pump working fluid volumes, and both are equalized using separate expansion compensation to the pump inlet so that no differential pressure exists across the seal. This is accomplished by equalizing the electric motor to the pump inlet through an expansion diaphragm in the motor and by separately equalizing the closed hydraulic system in the pump, which is also equalized to the pump inlet by the working fluid sub-chambers.
To further compensate for the potential loss of fluid from the rotating seal, a make up valve may be used between the pump inlet and the well bore to introduce make up fluid through a filtered inlet. The valve would be spring loaded to open when the differential pressure between the pump inlet and the well bore indicates the hydraulic system requires more fluid to operate properly. The working fluid must be compatible with the well fluid, such as in the case where hydraulic oil is used as the working fluid in an oil well, or a water based fluid is used in a water well.
Another common problem in some applications is the diffusion of gas across the pumping membrane from the well fluid. This occurs when hydrogen, carbon dioxide or hydrogen sulfide are present in significant quantities. To eliminate these gasses from the system, a gas trap may be used. The gas trap consists of a small orifice connecting a rigid chamber located at the highest point in the working fluid sub-chamber to the working fluid sub-chamber. A spring loaded check valve is located at the highest point in the rigid chamber, and is set to open at a pressure slightly higher than the system switch pressure. When the system cycles between high and low pressure, gas will accumulate in the rigid chamber by passing through the small orifice under the influence of gravity. Once in the rigid chamber, the gas will exert pressure on the relief valve when the system switches from high to low pressure. When sufficient gas has accumulated, the relief valve will open and allow the gas to escape to the pumped fluid and out of the closed hydraulic system. Two gas traps may be required, one in each working fluid sub-chamber. A semi-permeable membrane can also be used in place of the check valve.